The present invention relates to the field of signal processing in preparation for encoding or decoding of Manchester-encoded signals, and more particularly, the invention concerns a retiming decoder/encoder which operates to decode or encode a Manchester signal waveform which is tailored to a particular transmission protocol.
Manchester encoding is a well-known technique in which data and data clock components are combined into a single encoded waveform. The encoded waveform is divided into time slots of equal duration commonly called data cells during each of which one binary digit (bit) of information is conveyed. The state of the bit is indicated by a transition in the waveform occuring at the center or mid-cell point of the data cell. The direction of the transition indicates the value of the bit. At least one signal transition per data cell occurs, providing a component in the frequency domain centered at twice the bit occurence rate.
The digital nature of the Manchester code provides the usual preservation of data information in the face of communication channel corruption by noise and other transmission effects. The timing component of the waveform eliminates the need for two physically separate transmission channels, one for data and one for timing.
As is known, a transmission standard, MIL-STD-1397B, Type E (NAVY), has been promulgated which provides for two-way information transfer in a single bi-directional transmission channel. The transmission scheme has three states: logic 1, logic 0, and an idle state. The protocol of this standard imposes a frame format in which up to a given number of bits are transmitted as an integral unit from an information source on the transmission line. A "synch" bit begins each frame. A frame may have three or more bits in addition to the synch bit. Each frame size is variable, and the end of a frame is indicated by the presence of the idle state.
An essential element of a communication system employing the data transmission and protocol approaches just described is an element which is able to convert a Manchester-encoded waveform (normally a three-state signal) to a two level digital waveform and which can convert a two level digital waveform into a Manchester waveform. Such an element is commonly referred to as a "codec", which is a shortened form of the term coder/decoder. As is conventional, codecs are important components of data links between computers and peripherals. Frequently, such data links must convey information over relatively great distances, thereby often employing a Manchester-encoded transmission channel. In such an application, a codec decodes data into a local format from the Manchester-encoded format and encodes locally-formatted data into the Manchester format.
Usually, the decoder portion of the codec includes a retiming provision through which the received Manchester signal is improved by reestablishing the relative timing of waveform transitions in which, prior to decoding.
Presently, most codecs which have provision for waveform retiming utilize discrete parts, some of which are analog and which are therefore susceptible to performance variations due to temperature and device tolerances. Additionally, it is difficult and expensive to implement analog components in large, predominantly digital, monolithic integrated circuit form. Many of these codecs also exhibit substantial throughput delays. Codecs which are designed to operate according to MIL-STD-1397B, Type E frequently fail to provide all control signals necessary to the local interface end of the transmission interface. Last, many of these latter codecs fail to synchronize the decoded signal with any local clock.